1. Field of the Invention
The present invention relates to wastewater treatment systems and methods, and, more particularly, to such systems and methods for wastewater treatment that are nonchemically based.
2. Description of Related Art
Wastewater treatment via “natural” means, i.e., without the addition of chemicals, has been accomplished with the use of aquatic and emergent macrophytes (plants) that, in concert with the attendant microorganisms and macroorganisms associated with macrophyte roots and stems, substantially mineralize biodegrade organic materials and substantially remove certain excess nutrients, such as nitrogen and, to a lesser extent, phosphorus. These macrophytes have typically been located in artificial marshlands, also known as constructed wetlands, which are designed for gravity flow. A negative aspect of such systems is that they are very land-intensive, requiring roughly on the order of 100 times as much land area as a conventional treatment plant, or, in terms of capacity, as much as 30-40 acres per 106 gallons of wastewater treated per day unless other treatment processes are incorporated into the constructed wetlands.
Subsurface-flow wetlands, which comprise aquatic plants positioned above a gravel filter, are also known for use in wastewater treatment. These systems have been shown to frequently fail, however. Failure is manifested as the upstream gravel tends to become clogged with biosolids, permitting the influent to bypass the clogged region and pass substantially untreated to a downstream region. Additionally, surface wastewater is a breeding ground for disease vectors and nuisance insects. Ultimately the gravel becomes so clogged that design wastewater treatment is substantially compromised. Plants also appear to have little treatment role in subsurface flow wetlands because the plant root systems are inhibited by conditions in the gravel filter from growing sufficiently long to extend into the gravel, and thus have minimal contact with the influent.
Several varieties of aquatic and emergent macrophytes are known to be used in wetland and aquatic wastewater treatment systems, including, but not limited to, cattails, bulrushes, sedges, and water hyacinths. In wetland treatment systems these plants may be packed in unlined or lined trenches or basins filled with a granular porous medium such as gravel or crushed stone. It has also been suggested to use recycled, shredded scrap tires in the place of the gravel. Another suggested wetland system variant is to place a semipermeable barrier between a lower level into which effluent enters and the plant root system for directing the wastewater flow across the entire plant bed.
In yet another variant, floating aquatic macrophytes, typically water hyacinths, are placed in shallow lagoons where plant roots, with attendant microorganisms and macroorganisms, extending into the water column are a principal design treatment mechanism. Although this root zone treatment method can provide advanced secondary treatment effluent, its application is limited by climate and available sunlight to approximately 5% of the United States. The large treatment footprint of water hyacinth treatment systems prohibits enclosure in greenhouses for almost all economically viable applications.
It is also known to combine plant root zone treatment with conventional activated sludge technology. The principal advantages of combining root zone treatment with activated sludge are improved nutrient removal capability over root zone treatment alone and improved treatment stability in small, activated sludge treatment systems. Among the problems encountered with root zone/activated sludge technology is that the clarifiers employed do not scale well when the size of the system is reduced beyond a certain point. In addition, operator qualifications are high for activated sludge systems, adding to the expense of running the system. Root zone/activated sludge technology has been known to digest in situ a large fraction of the biosolids produced and maintained within the treatment system, thereby reducing system biosolids yield. The mechanism for yield reduction is thought to be the retention of biosolids flocs on plant roots with subsequent consumption and mineralization of flocs by the invertebrate community attendant to the root zone. Reduction of yield is desirable only to a certain point, however. As reactors in series are added, thereby increasing biosolids contact with the root zone, yield may be reduced to the point where an insufficient quantity of biosolids remains to be recycled from the clarifier to the reactors in series. Lack of recycled biosolids substantially degrades the treatment performance of the activated sludge treatment element. This design trap is inherent to root zone/activated sludge treatment systems.
Preliminary studies have been performed on various aspects of the present invention by the inventors and other colleagues, and these have been reported in “Final Report on the South Burlington, Vt. Advanced Ecologically Engineered System (AEES) for Wastewater Treatment,” D. Austin et al., 2000; and “Parallel Performance Comparison between Aquatic Root Zone and Textile Medium Integrated Fixed Film Activated Sludge (IFFAS) Wastewater Treatment Systems,” D. Austin, Water Environment Federation, 2001; both of these documents are incorporated herein by reference in their entirety.